We present a system for secure telephone conferences (stc) over the internet. The system ensures participant authentication via x.509 certificates, such that every participant of a conference is informed about every other participant. Also, all signaling and media data are encrypted, to ensure confidentiality. The system builds upon the open source telephone server asterisk and standard IP softphones. Those software products are used unaltered. Stc client and server processes reside with softphones and server, respectively, to realize secure conferences. Experiments with our prototype show that the additional network and processor load is low, and that the system scales well for more than 10 participants.